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The development and application of gallium nitride–heterojunction field-effect transistor (GaN–HFET) has been actively researched. Because GaN–HFETs have advantages in high-frequency operation, it is possible to downsize power converters by increasing the switching frequency. As the switching frequency rises, the switching loss increases in proportion to the frequency; therefore, application of a soft-switching method is needed to decrease the losses and heat generation. The resonant power conversion circuits represented by the class Phi-2 inverter have soft-switching feature; therefore, these circuits realize low-switching-loss operation even in high-frequency regions. However, the operation of these resonant circuits depends on the load conditions, generally. Thus, it is important to investigate the characteristics of the inverter for various load conditions. In this paper, we investigate the characteristics of the non-isolated and isolated class Phi-2 inverters by changing the load resistance and the duty ratio of the boost circuit connecting after the class Phi-2 circuit, which is operated at 13.56 MHz. We also developed a protection system during abnormal operations such as an open circuit. From experimental results, the class Phi-2 inverter has superior characteristics under various load conditions and changes. Moreover, we confirmed the effectiveness of the abnormal operation stop system.

In this paper, we present a comprehensive reviewand discussion of the state-of-the-art device technology and application development of GaN-on-Si power electronics. Several device technologies for realizing normally off operation that is highly desirable for power switching applications are presented. In addition, the examples of circuit applications that can greatly benefit from the superior performance of GaN power devices are demonstrated. Comparisonwith other competingpower device technology, such as Si superjunction-MOSFET and SiC MOSFET, is also presented and analyzed. Critical issues for commercialization of GaN-on-Si power devices are discussed with regard to cost, reliability, and ease of use.

We investigate the dc, electroluminescence (EL) and trapping properties of gate-injection transistors without and with the pdrain terminal connected to the drain (called GITs and HD-GITs respectively) and with and without field-plate. Our results indicate that: (i) the dc characteristics are not influenced by the presence of the pdrain terminal and of the field plate; (ii) EL measurements indicate that GITs and HD-GITs have comparable hot-electron density and energy. (iii) When submitted to OFF-state, all devices show similar dynamic-RON, independently of the presence of pdrain and field plate; (iv) on the other hand, under semi-ON trapping conditions substantial differences are observed. Specifically, the HD-GITs have a significantly lower dynamic-R ON compared to GITs; a further improvement is obtained through the use of a field plate. (v) Transient R ON measurements indicate that the traps filled are the same both under OFF-state and semi-ON conditions. The activation energy E a of this trap is E a ∼0.8 eV, i.e. it could be ascribed to deep levels related to C N , as suggested also by pulsed measurements carried out under external illumination. Based on the original results collected within this paper, we conclude that the difference in dynamic-R ON shown in semi- ON by GITs and HD-GITs is related to the injection of holes from the pdrain terminal that reduces the impact of hot-electron trapping processes.

This paper investigates the trapping mechanisms in gate-injection transistors (GITs) without and with a pdrain electrode, referred to as GITs and Hybrid-Drain-embedded GITs (HD-GITs), respectively, used to inject holes and reduce charge trapping effects. We compare the two sets of devices under both the OFF-state and semi- ON state, to investigate the role of hot electrons in favoring the charge trapping. The analysis is based on combined pulsed characterization, transient measurements, and electroluminescence (EL) characterization. We demonstrate the following relevant results: 1) GITs and HD-GITs have comparable and negligible dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}$ when trapping is induced in the OFF-state; under semi- ON state conditions, GITs suffer from significant dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}$ , while HD-GITs show no additional trapping with respect to OFF-state; 2) EL characterization carried out until ${V}_{\mathsf {DS}}= 500$ V in semi- ON conditions shows comparable features, suggesting that the electric field and hot-carrier density are similar in GITs and HD-GITs. This result indicates that the hot-electron trapping rate is identical for the two sets of samples, so the difference observed in dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}$ must be ascribed to a different detrapping rate; 3) transient ${R}_{ \mathrm{\scriptscriptstyle ON}}$ measurements indicate that the traps filled in the OFF-state conditions are the same as those filled by hot electrons in semi- ON, with ${E}_{a}= 0.8$ eV (possibly ${C}_{N}$ ); and 4) EL analysis under constant bias indicates that in GITs there is a time-dependent increase in the luminescence signal at the drain terminal, when the devices are biased with ${V}_{\mathsf {DS}}= 300$ V. Such effect, indicative of hot-electron trapping, is not observed in HD-GITs. Based on the experimental evidence collected within this paper, we conclude that—in the semi- ON state—the main difference between GITs and HD-GITs consists in a faster detrapping rate of hot electrons, achieved through hole injection from the pdrain terminal.

This paper reviews the state-of-the-art technologies of the normally-off GaN Gate Injection Transistor (GIT) and its applications such as inverter for motor drive and Power Factor Collection (PFC). Fundamental performances such as the current-collapse-free operation and the excellent switching performance are reviewed.

The simple and high-accuracy equivalent circuit model for GaN-GIT bi-directional switch have been constructed by eliminating the gate resistance dependence from the capacitance parameters and employing the parallel gate resistance circuit with the diode. By the new methods, the calculated waveforms, switching losses and switching times agree well with the experimental ones, and the accuracy between the calculations and the experiments are more than 90%. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we discuss the method of reducing peak voltage between drain and source, it is occurred during the start of the class Phi-2 inverter. The class Phi-2 inverter is resonance inverter circuit with only one switching device, it is able to operate at high frequency. During start of this circuit, 150% peak voltage is impressed on switching device in comparison with steady state, so increase of voltage stress is concerned. Especially, if we use the class Phi-2 inverter as DC-DC converter with ON/OFF control, this problem is more remarkable. In this study, we applied the soft-start method to switching device GaN-HFET (GaN-Hetero-junction Field Effect Transistor) and investigated reduction of peak voltage during start of the circuit. From simulation and experimental investigation, the peak voltage has been reduced by 75%, we confirmed the stress of the switching device is reduced by proposed soft-start method. Therefore, it is possible to operate under higher input voltage of class Phi-2 inverter even if low withstand voltage semiconductor is applied.

Trade-off between threshold voltage and specific on-resistance is successfully overcome in a diode-integrated SiC MOSFET by improving the design of n-type epitaxial channel layer and p-type body region. This new design features enhanced transconductance, hence low on-state resistance, while retaining high threshold voltage. Obtained specific on-resistance of the fabricated 1200V SiC DioMOS is among the lowest achieved for SiC MOSFETs including trench devices. The transconductance enhancement is also demonstrated to be effective in increasing the turn-on switching speed, thus contributing to higher efficiency in power switching systems with reduced conduction and switching losses.

In recent years, the development and application of GaN-HFET(Heterojunction Field Effect Transistor) has become actively. As GaN-HFETs have advantages in high frequency operation, it is possible to down size power converters by rising switching frequency. In this paper, we investigate the load fluctuation of an isolated DC-DC converter with the class Phi-2 inverter circuit using GaN-HEFT, which is operated at 13.56 MHz.

Highly efficient three-phase to three-phase matrix converters using Gallium nitride (GaN) bidirectional switches with both high current and high breakdown voltage are demonstrated. The GaN switch with dual gates works as a bidirectional switch by a single device, while a conventional bidirectional switch consists of four devices by two Insulated Gate Bipolar Transistors (IGBTs) and two diodes. In addition, the GaN bidirectional switch is also free from the voltage offsets for the current conduction so that the GaN-based matrix converter enables small size and highly efficient AC/AC conversion. Improvement of the device performance including the introduction of the recessed gate enables the low on-state resistance with stable operation free from current collapse. The maximum drain current reaches 100 A together with the breakdown voltage of 1340 V. The fabricated three-phase to three-phase matrix converter exhibits the maximum conversion efficiency of 98% at 1 kW output power with the expectation that the maximum output power can reach 10 kW or more by the high current device.

Reliability of a GaN-based Hybrid-Drain-embedded Gate Injection Transistor (HD-GIT) under continuous switching operation is investigated to extract the lifetime under a practical switching application. Switching lifetimes are deduced by varying ambient temperature, switching voltage and switching current to obtain their corresponding acceleration factors. Based on the obtained factors, we determine Switching Safe Operating Area (SSOA) that is proposed recently to define the I ds -V ds area inside which the device can be switched safely. To our knowledge, this is the first demonstration of SSOA for a GaN power transistor. We estimate the switching lifetime when the HD-GITs are employed in 3 kW totem-pole power factor correction (PFC) circuit under a typical usage condition. The estimated lifetime is 24 years, which we believe is sufficiently long.

We propose a normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm2 and high off-state breakdown voltage of 1.7 kV. P-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables high threshold voltage of 2.5 V and stable switching operations. The employed p-type gate does not give any concern of the gate instability. Note that formation of carbon doped insulating GaN layer formed on p-GaN well layer underneath the channel suppresses the punch-through current at off-state between the source and drain, which enables good off-state characteristics. The fabricated high-current vertical transistor achieves successful fast switching at 400V/15A. We also propose a novel vertical GaN-based junction barrier Schottky (JBS) diode with trenched p-GaN region on a bulk GaN substrate. A specific differential on-resistance of the GaN JBS diode is 0.9 mΩ·cm2 while keeping high breakdown voltage of 1.6 kV. These results indicate that the demonstrated vertical GaN devices are very promising for future high power switching applications.

In recent years, as GaN power transistors come into widespread use as switches for power converter applications, it is all the more crucial and inevitable to guarantee their reliability. In this chapter, we discuss how to evaluate the robustness of GaN power transistors. In the switching of GaN power transistors, they can be subject to the so-called current collapse that is a specific phenomenon for GaN in which the ON state resistance is increased once the device is exposed to a high voltage. Since the current collapse induces instability of the device in the form of the increase in the temperature, non-uniform internal electric field distribution and so on, it may lead to the reliability issue. Therefore the robustness of GaN transistors should be examined under switching operation besides the conventional reliability tests standardized for Si power transistors. Since current collapse is crucially dependent on the drain current – voltage locus curve during the switching event, the switching reliability of GaN transistor depends on the switching locus. Accordingly a concept of Switching Safe Operating Area (SSOA) is proposed to define the switching conditions wherein the device can be switched safely. As an example, we define the SSOA for our Hybrid-Drain-embedded Gate Injection Transistor (HD-GIT) that is now commercially available. Furthermore we propose the long-time SSOA (lSSOA) in which we guarantee the robustness of HD-GIT under long-time switching operations (e.g., 10 years). The proposed method for confirming the robustness of GaN power transistors can be utilized to estimate the devices’ lifetime when they are employed in a given switching application.

In this paper, high current and high voltage AlGaN/GaN metal-insulator-semiconductor (MIS) heterojunction field-effect transistors (HFETs) on Si are demonstrated. The devices exhibit a drain current of 20 A as well as a breakdown voltage of 730 V, serving normally-off operations. Stable interfacial characteristics free from the hysteresis in the transfer characteristics are enabled by the introduction of AlON gate insulator. A recessed gate structure formed by epitaxial regrowth of AlGaN over the grooved AlGaN/GaN heterojunction successfully reduces the on-state resistance and eliminates the processing damage on the surface of the grooved structure. Note that an oxygen annealing followed by the deposition of AlON shifts the threshold voltage V th to positive side. The resultant switching performance by the 20 A / 730 V AlGaN/GaN MIS-HFET is very fast with dV/dt of 78 V/ns and 169 V/ns for turn-on and turn-off transitions, respectively, indicating that the proposed MIS-HFETs are very promising for practical power switching applications.

We have demonstrated hysteresis-free recessed gate AlGaN/GaN metal-oxide-semiconductor heterojunction field-effect transistor (MOS-HFET) by implementing AIGN gate insulator and selective AlGaN regrowth technique. High thermal stability and excellent electrical properties of AIGN gate dielectrics will provide a large process window for further optimization of AlGaN/GaN MOS-HFET.

Reliability tests on 600V-rated GaN-based normally-off hybrid-drain-embedded Gate Injection Transistor (HD-GIT) are performed. High-temperature reverse-bias test on HD-GIT reveals that the lifetime is dependent on the leakage current before the reliability test. Acceleration factors for the temperature and reverse bias voltage are extracted. Based on the obtained results, devices are designed so that their lifetimes under Vbs=480V (80% de-rated) at 80°C are longer than 1000 years, which are long enough for most conventional power converter applications. Dynamic high-temperature operation life test is also performed, under which their DC characteristics are within their normal range up to 3600h. In addition, HD-GITs are within their normal range under the similar reliability tests as applied for Si-based power transistors. The physical mechanisms for the reliability of HD-GIT are discussed. We conclude that the OFF-state reliability is explained by the so-called percolation degradation model, and that the hole injection from the p-type GaN embedded in the drain plays an important role to suppress the degradation under hard-switching operation test.

In this paper, we describe compact isolated gated drivers with Drive-by-Microwave (DBM) technology, which can provide isolated gate signal and power all together by a microwave wireless power transmission. The fabricated GaN gate driver successfully drives a GaN power switching device without an isolated voltage source. Furthermore, an ultra-compact GaN 3x3 matrix converter with DBM technology is presented, which is composed of a GaN-GIT (Gate Injection Transistor) bi-directional power switches integration chip, electro-magnetic resonant coupling based isolated dividing couplers in printed circuit board to reduce the complicated gate lines, and an one-chip and low consumption GaN/Si DBM driver chip for 9 bi-directional power switch control. The proposed GaN 3x3 matrix converter for 5.0kW is overwhelmingly compact such as 25mm x 18mm because of GaN power device integration and the DBM technology. Furthermore the power consumption for all divers is very low as 2.0W due to the isolated microwave power source sharing.

We propose a novel concept of thin and compact CPV modules in which submillimeter solar cells are directly attached to lens arrays without secondary optics or an extra heat sink. With this small cell size, the optical path length of the module can be brought down to one-twentieth that of conventional CPV modules. To achieve precise alignment of the microsolar cells at the lens focal points, we have developed a fluidic self-assembly technique that utilizes surface tension. This novel CPV module with triple junction microsolar cells demonstrated an efficiency of 34.7% under sunlight in the particular measured condition.

The radiated emission from the GaN-HEMT switching converter is investigated by two-dimensional electromagnetic-probe scanning. By the scanning, the single peak spectrum due to the ringing of the switching waveforms is obtained. When the supply voltage of the converter Vcc is varied, this peak frequency of the spectrum of the electromagnetic field is shifted. This peak shift is thought to be due to the change of the capacitance of the depletion region in the GaN-HEMTs. Therefore, we propose the equivalent circuit model for the radiated emission employing the resonant frequency between the output capacitance of the GaN-HEMTs and the parasitic inductance of the circuit wire Lline. The calculated results at Lline of 1.2 nH agree well with the experimental results. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Device technologies of SiC MOSFETs have nearly matured to the level of mass production and one of the remaining tasks is to serve better solutions in view of both costs and performances for practical systems. Elimination of external reverse diodes in inverter circuits is one of the solutions, by which total area of the SiC chips is greatly reduced leading to lower material cost. A DioMOS (Diode in SiC MOSFET) successfully integrates the reverse diode without any increase of the chip size from the original MOS transistor by utilizing an n-type epitaxial channel under the MOS gate for the reverse conduction path of the diode. The basic concept of the DioMOS has been proposed [1]; meanwhile, further reduction of the on-state resistance together with confirmation of high-speed switching is necessary for its application in power switching systems. In this paper, low on-state resistance (Ron) of 40mΩ and blocking voltage (BVds) of 1700V as well as improved switching performances of DioMOS are demonstrated. The measured results suggest DioMOS to be satisfactory for practical use.

GaN-based normally-off Gate Injection Transistors (GITs) with p-type gate over AlGaN/GaN heterojunction are fabricated on bulk GaN substrates. Thickness of insulating GaN buffer layer is increased up to 16 μm for the presented device from 5 μm for conventional GITs on Si. The thick buffer reduces the parasitic output capacitances, which enables fast turn-off switching. The thick buffer and the use of bulk GaN substrate help to improve the crystal quality of AlGaN/GaN so that the sheet resistance is reduced. Improved crystal quality together with reduced trap density successfully suppresses the current collapse up to 1 kV or higher of the applied drain voltage. The resultant R on Q oss (R on : on-state resistance, Q oss : output charge) as a figure-of-merit for high speed turn-off switching is reduced down to 940 mΩnC that is one third from that of GITs on Si. The resultant turn-off dV ds /dt reaches as large as 285 V/ns that is twice higher than reported values by GITs on Si.

A normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm2 and high off-state breakdown voltage of 1.7 kV is presented. P-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables high threshold voltages of 2.5 V and stable switching operations. Note that formation of carbon-doped insulating GaN layer formed on p-GaN well layer underneath the channel suppresses the punch-through current at off-state between the source and drain, which enables good off-state characteristics. The fabricated high-current vertical transistor achieves successful fast switching at 400V/15A. These results indicate that the demonstrated vertical GaN transistor is very promising for future high power switching applications.

The status and challenges in the development of GaN power devices are reviewed. At present, normally-off gate injection transistors (GITs) on Si are commercially available. The updated structure known as a hybrid-drain-embedded GIT provides superior reliability that contributes to the stable operation of compact power switching systems with high efficiency. The fabricated vertical GaN transistor on GaN as a future challenge demonstrates extremely low specific on-state resistance and high breakdown voltage. Metal-insulator-semiconductor-gate GaN transistor is also a technical challenge for faster switching, since it would give greater freedom of gate driving as a result of both high threshold voltage and widened gate voltage swing. Normally-off operation free from hysteresis in the current–voltage characteristics is confirmed in a recessed-gate AlGaN/GaN heterojunction field effect transistor using AlON as a gate insulator. Fast switching characteristics are experimentally confirmed for both of the newly developed GaN devices, indicating their great potential for practical use.

In this paper, we investigated the effects of the post-deposition annealing (PDA) on the threshold voltage (V th) of AlGaN/GaN MOS heterojunction field-effect transistors with atomic-layer-deposited Al2O3 using H2O vapor or oxygen plasma as the oxidant. By PDA, the V th shifts positively with the V th variations depending on the oxidants. The capacitance–voltage measurements reveal that the V th variation is attributed to the differences in the initial fixed charge density in the Al2O3 or/and the Al2O3/AlGaN for both oxidants.

This paper reviews the recent activities for normally-off GaN-based gate injection transistors (GITs) on Si substrates and their application to inverters. Epitaxial growth of the AlGaN/GaN heterostructures with good crystallinity over 200-mm Si substrates with eliminated bowing enables low-cost fabrication of GaN devices with high breakdown voltages. A novel normally-off GaN transistor called as GIT is proposed in which hole injection from the p-type AlGaN gate increases the drain current with low on-state resistance by conductivity modulation. The low on-state resistance in GaN-based devices greatly helps to increase the efficiency of power switching systems. A GaN-based three-phase inverter successfully drives a motor with high efficiency of 99.3% at a high output power of 1500 W. The presented GaN-based devices are expected to greatly help saving energy in the future as an indispensable power switching system.

Ion implantation technique can be applied for planar isolation of AlGaN/GaN heterojunction field-effect transistors (HFETs), which enables high-density integration of the power switching transistors. So far, the reported isolation using ion implantation for GaN devices has never maintained high isolation voltages after high-temperature processing over 800 °C which is commonly used for the fabrication. In this paper, we present detailed analysis and mechanism of thermally stable isolation of GaN devices by Fe ion implantation keeping high breakdown voltage between the devices after high-temperature annealing. Ion species forming deep levels at atomic sites in GaN are examined by using first-principle calculation prior to the experiments. The calculation indicates that the Fe ions stay at Ga sites with deep levels in GaN. The following experiments using various ion species well agree with the aforementioned predictions, where implanted regions by other ions than Fe exhibit reduction of the resistivity after high-temperature annealing to recover the processing damage by the ion implantation. As a result, it is experimentally found that Fe is the only choice to serve high resistivity after the annealing. The Fe ion implantation enables high breakdown voltage of 900 V after the annealing at 1200 °C. This technique is indispensable to enable monolithic integration of the lateral AlGaN/GaN HFETs for high-voltage power switching systems.

This chapter describes an enhancement-mode (E-mode) GaN transistor named as Gate Injection Transistor (GIT) and various technologies to improve the performances. The operation principle of the GIT as well as its state-of-the-art DC and switching performances is described after summarizing reported E-mode GaN power transistors. Status of the reliability including Current collapse is summarized, followed by the results of the application of the GIT to practical power switching circuits targeting at high efficiencies. Future technologies to improve the performances and to extract the potential are also described.

In this paper, we present a state-of-the-art integrated GaN power IC capable of operating in a high frequency (MHz) regime. This realizes system size reduction, 60% maximum, of a power IC. The IC consists of two output power transistors (PT) and two gate drivers (GD). The key devices in the IC are normally-off gate injection transistors (GITs) for PT and GD and a normally-on hetero-junction field effect transistor (HFET) for GD. Novel local control of carrier concentration of an identical 2 dimensional electron gas (2DEG) at an AlGaN/GaN interface which made integration of the transistors with such a large threshold voltage difference possible is described. A specially developed post-passivation interconnection process giving low parasitic components is also described. The IC applied to a 12V–1.8V DC-DC converter shows high frequency switching operation well beyond the limit of Si pointing to future improvement in consumer electronics power supply systems.

The switching waveforms and losses of a GaN gate injection transistor (GIT) bidirectional switch, a type of four-terminal device, were analyzed for the first time using an equivalent circuit model. By applying a three-terminal model to the equivalent circuit model of the GIT bidirectional switch and by using the waveforms of the chopper circuit, the parameters were derived with high accuracy. Furthermore, gate resistance dependence was added to the input capacitance component connected to the gate terminal in order to contain the influence of the gate structure of the GIT. It was confirmed that the calculated switching waveforms and losses agree well with those of the experimental values with over 90% accuracy, even in cases where circuit conditions for circuit voltage, load current, and gate resistance were varied.

We obtain an equivalent-circuit-model for GaN gate-insulated-transistor (GIT) bi-directional switches (BDSWs) which consists of one current source and three capacitances. Employing this model for the chopper circuit simulation, the gate resistance dependence is included into the input capacitance of CG1S2 and CG1S1 in order to improve the accuracy. By including this influence of the gate resistance, the calculated waveforms and the estimated switching losses agree well with the experimental ones with accuracy of over 90% when the circuit conditions are varied. This gate resistance dependence is considered to be caused by the hole injection at the gate region. By using our proposed model, this equivalent-circuit-model for GIT BDSWs can be treated with like a simple single-gate MOSFET model (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Recent progress in GaN-based power electron devices and advanced thermal management techniques for high performance GaN power devices are described. Recent advances in high performance GaN-based transistors are reviewed. The reduction of junction temperatures in low-pressure direct liquid cooling of GaN power devices for high-power and high-voltage applications are demonstrated using perfluorocarbon and perfluoroether as a working fluid, for the first time.

This paper reviews novel design of epitaxial structure for GaN-based power devices taking advantages of the material's unique polarization. This can be called as polarization engineering which enables low on-state resistances and high breakdown voltages. AlGaN/GaN superlattice and polarization-matched InAlGaN quaternary alloy capping layers effectively reduce the series resistance of AlGaN/GaN heterojunction field effect transistors (HFETs) by reducing the potential barriers above the heterojunction. Natural super junction (NSJ) model is proposed, which well explains the limitless increase of the breakdown voltages of GaN transistor by the extension of the gate-drain spacing. This model is applied to diodes with multi channels of AlGaN/GaN resulting in low on-state resistances and high breakdown voltages. The presented polarization engineering is very promising for future GaN power devices with the improved performances.

The impacts of inserting ultrathin oxides into insulator/AlGaN interfaces on their electrical properties were investigated to develop advanced AlGaN/GaN metal–oxide–semiconductor (MOS) gate stacks. For this purpose, the initial thermal oxidation of AlGaN surfaces in oxygen ambient was systematically studied by synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) and atomic force microscopy (AFM). Our physical characterizations revealed that, when compared with GaN surfaces, aluminum addition promotes the initial oxidation of AlGaN surfaces at temperatures of around 400 °C, followed by smaller grain growth above 850 °C. Electrical measurements of AlGaN/GaN MOS capacitors also showed that, although excessive oxidation treatment of AlGaN surfaces over around 700 °C has an adverse effect, interface passivation with the initial oxidation of the AlGaN surfaces at temperatures ranging from 400 to 500 °C was proven to be beneficial for fabricating high-quality AlGaN/GaN MOS gate stacks.

In this paper, the current–voltage (I–V) characteristics of a 600-V-class normally off GaN gate injection transistor (GIT) from 25 to 200 °C are analyzed, and it is revealed that the drain current of the GIT increases during high-temperature operation. It is found that the maximum drain current (I dmax) of the GIT is 86% higher than that of a conventional 600-V-class normally off GaN metal insulator semiconductor hetero-FET (MIS-HFET) at 150 °C, whereas the GIT obtains 56% I dmax even at 200 °C. Moreover, the mechanism of the drain current increase of the GIT is clarified by examining the relationship between the temperature dependence of the I–V characteristics of the GIT and the gate hole injection effect determined from the shift of the second transconductance (g m) peak of the g m–V g characteristic. From the above, the GIT is a promising device with enough drivability for future power switching applications even under high-temperature conditions.